1. Field of the Invention
The present invention relates to objectives, particularly projection objectives, and to a method of fabricating an objective.
2. Discussion of Related Art
For the fabrication of components such as circuits and the like in microelectronic applications, microlithographic techniques are employed making use of high-accuracy objectives particularly in the field of projection illumination systems. The ever increasing miniaturization makes it necessary to resort to ever smaller wavelengths for the projection light employed in projection illumination systems, now resulting in projection light in the spectral range of extreme ultraviolet (EUV) light being employed. The wavelengths involved are in the range of very few to a few 10 nm, particularly in the range of about 13.5 nm. In addition to higher demands on the accuracy this also involves the use of catoptric systems.
In general, however, with projection objectives which may also work with longer wavelength projection light, there is the requirement that the projection objectives should ensure highly exact images, and correspondingly, aberrations should be maintained at extremely low levels.
Aberrations may result from the design of the projection objective, i.e., the arrangement and orientation of the various optical elements such as refractive optical elements, in other words lenses, or reflective optical elements such as mirrors. In addition, aberrations due to fabrication and/or material defects of the optical elements, or the optical surfaces defining them, should also be particularly taken into account. Some aberrations that should be particularly accounted for are misshapings, the so-called form errors both for lenses and for mirrors that involve departures from the ideal shape or surface because of production tolerances. To be additionally taken into account, especially with lenses, are material defects as may exist, for example, due to localized fluctuations in the properties of the material, such as, for example, the refractive index.
One method to correct such aberrations due to fabrication or material defects is discussed in U.S. Pat. No. 6,268,903 B1 in which the surface of the optical elements is remachined to correct for aberrations due to fabrication and/or material defects. Such remachining methods are also described in the German paper by C. Hofmann et al entitled “Nanometer Asphären: wie herstellen und wofür” in the magazine “Feinwerktechnik und Messtechnik,” 99 (1991), 10, pages 437-440. Both the content of the above-mentioned article by C. Hoffmann et al and that of U.S. Pat. No. 6,268,903 B1 (or corresponding DE 696 14 007) are incorporated as content and subject matter of the present application by reference.
DE 103 60 414 BI describes in relation to an EUV projection objective how the ready assembled and adjusted EUV projection objective is gauged and, subsequently how the surface of one or more mirrors can be remachined so that certain imaging properties of the projection objective are improved. This, however, involves the problem that the mirrors are usually coated and thus that remachining could cause localized detriments to the reflectivity of the mirrors. This problem is taken into account in DE 103 60 414 by only one mirror being corrected before coating. The content and subject matter of DE 103 60 414 is likewise incorporated by reference in the present application. In addition, U.S. 60/578522 or PCT/EP2005/005930 discuss which optical elements are particularly suitable for correction. According to these documents, at least two optical elements arranged in different sections of an objective are to be corrected, the sections being divided by pupil planes or intermediate image planes disposed in-between. The two corrected optical elements are to be arranged in sections in which the absolute value of the difference in the section numbers is an odd number.
Although known prior art already makes it possible to achieve a very good correction of aberrations due to fabrication and/or material defects there continues to be a need to attain a compromise between the conflicting interests of achieving an optimized correction on the one hand and on the other minimizing the expense involved.